High frequency current transformer



H. WEHRLIN Sept. 17,` 1940.

Filed Feb. 18, 1957 HIGH FREQUENCY CURRENT TRNSFORMER Patented Sept. 17,1940 UNITED STATES IPii'rizin OFFICE HIGH FREQUENCY CURRENT TRANS-FORMER COmPaIly Application February 18, 1937, Serial No. 126,531 v InGermany February 18, 1936 Claims.

It is well known to employ current transformers when measuring highfrequency currents by means of arrangements' of the kind wherein thepoints of reading and those of measurement are 5 not identical. Anotherreason why current transformers should be used is that the inherentcapacity of the instruments employed would be too great if they weredirectly included in the line, whereas the inherent capacities of thecurrent transformer allow of being rendered much smaller.

The invention lproposes to provide high frequency current transformerswith cores made of high frequency iron.

Under certain conditions the ratio of transformation is equal to that ofthe number of turns leakage and watt power being required here to besuiiiciently small.

In arrangements as provided by the invention the spatial dimensions andthus the inherent capacities and leakage are reduced by furnishing thesecondary coil with a core of high frequency iron.

Some embodiments of the invention are disclosed hereafter by way ofexample, reference being had to the accompanying drawing in which Fig. 1is a partially sectioned view of one form of the novel currenttransformer. Fig. 2 is a partially diagrammatic and partially sectioned'view showing a slight modification of the current transformerrepresented in Fig. 1 and a measuring equipment therefor. Fig. 3 is asectional view of a third modiiication of a current transformer asprovided by the invention. Fig. 4 is a schematic representationillustrating a novel mode of attaching a measuring equipment to acurrent transformer.

I denotes the primary conductor which may be a'tube, for instance. IIdesignates the toroidal 40 coil which as shown in Fig. l is providedwith a coreFe of high frequency iron. Coil II is carefully screened. Thescreening A arranged to such end is such that no short-circuitingeffects shall occur. For this purpose the screening A has acircumferential slot filled up with an insulating ring a. Coil II andscreening A are as a whole mounted on an insulating body K made of aceramic material, forinstance, and are in this way spaced from conductorI by a certain distance. seated in an annular groove of conductor I.Also a number of rings S may be provided in this way.

Body K in order to avoid air gaps is metallized where it contacts withconductor I and ring S.

55 The metallizing layer or coating is denoted `by P.

. tent.

Body K is supported by a spring ring S (Cl. F15- 358) The body K mayalso have a metal coating Y where it contacts with the screening A. Atube R, made of a ceramic material, for example, and provided insidewith a metal coating Q, is inserted over conductor I. The arrangement issuch that the thickness of the Wall of tube R and the distance betweentube R and screening A are in the inverse ratio of thedielectricconstants of the insulating material and of air. In this way puncture isprevented to the greatest possible ex- Tube R may be formed integralwith the body K. The screening A has a socket b by means of which a tubeT is secured thereto in order to screen a line L by which coil II isconnected to a measuring equipment E, D, M, Fig. 2. This equipmentcomprises a direct current indicator M and a thermo-electric couple E,connected in circuit therewith. If E and M are not comprised in onedevice, line L interconnecting E and M should be carefully screened by atube T', because otherwise this line may happen to receive highfrequency currents while no current is flowing through conductor I.Since the thermo-electric couple E becomes heated by a high frequencycurrent received by the line L the indicator M would be caused todeflect its pointer and thus to give a wrong indication. If a mostcareful screening should not be sufiicient to obviate this, then it willbe suitable to interpose a choke or a filter'chain D between E and M inorder to prevent high frequency energy received by L from kacting uponthe thermo-electricv couple E. A means of this kind would have to beprovided between A and E if the measuring equipment were to be protectedfrom currents received by line L. 'I'he arrangement D, E may becontained in a screening F.

-If short or very short waves are concerned then the body K is soconstructed that the dielectric between conductor I and transformer IIis constituted by air only. This feature, is likewise shown in Fig. 2.It will be seen that conductor I is here not contained in an insulatingtube such as tube R represented in Fig. 1. to the lowdielectric constanta small capacity bei tween transformer II and conductor I is obtained,that is, a capacity which is adequate to the high frequencies.

If the line interconnecting transformer II and thermo-electric couple Eor indicator M is longer than M4, A being the wavelength, then it isadvantageous to give part of the screening tube T a helical shape, asshown at t, in order that the line in this tube shall be coiled to forma choke, and this choke too may be fitted with a Core F6' Of In this waydue I high frequency iron. Between II and t the screening T is earthedby a conductor u and preferably at the current loop or potential node.If a definite operating wave is employed the choke at t may be connectedin parallel with a condenser C in order to form therewith an arrestercircuit tuned to the operating frequency.

The arrangement shown in Fig. 3 may be used if conductor I is of acomparatively large diameter. Conductor I is here made in two tubularparts. One of these carries a metal bolt B. The other part is fittedwith a metal block W, disposed therein. Coil Il'. is disposed within theother part, which thus also acts to screen it. The two parts of theconductor I are fastened to each other by a screw joint G, formed by athreadtprovided on bolt B and a thread provided in the block W. Careshould be taken that the two parts of conductor I do not contact witheach other, since otherwise short-circuiting effects may occur. The heatdue to the resistances constituted by the screw joint G may be removedby cooling fins Z with which conductor I is provided.

rIhe measuring equipment E, M if required to be exchangeable may beprovided with plug contacts e, e adapted to be inserted in sleeves o, o'to which coil II is connected over line L, as shown in Fig. 4. vCareshould be taken that when removing the device E, M from the plug sleeveso, o transformer 1I is automatically short-circuited since otherwisevery great high frequency voltages may occur at these sleeves. As arepresentative example of an arrangement adapted for this purpose acontact spring N is shown that tends to malte Contact with sleeve owhile being held out of Contact therewith by plug contact e. Further,the fbcurrence of high voltages in the case of wire breakage on thesecondary side can be obviated by a glow discharge tube H arranged tobridge over line L, and tube I-I may be arranged then to actuate analarm device.

The novel arrangement allows of predetermining by calculation the ratioof transformation except for about 10%.

Differences that may occur can in accordance with a further feature ofthe invention be compensated by varying the self-induction of thesecondary coil l1. This may be done as follows:

The core of the coil II may be made in several parts and these may bedisplaced with respect to each other in order to obtain the requisitereadjustment, or parts of the coil II may be displaced with respect toeach other, or a combination of these two modes may be employed. Also,the core of coil II may be provided with a bore intended to receive apin made of high frequency iron. The lself-induction is then varied byinserting this pin into the bore and by displacing it therein, ifnecessary, or by removing it from the bore.

What is claimed is:

i. A high frequency current transformer comprising a tubular primaryconductor having an intermediate portion of reduced diameter aroundwhich is positioned a secondary coil, and two portions of largerdiameter respectively extending in opposite directions from saidintermediate portion, said tubular primary conductor having an extensionencircling said secondary coil and acting as a screening therefor.

2. A transformer according to claim 1, wherein the primary conductor ismade in two parts and wherein a threaded electrically conductive bolt isemployed to interconnect these, the secondary coil being arranged tosurround this bolt.

3. A transformer according to claim l, wherein the primary conductor ismade in two'tubular parts spaced from each other.

4. A transformer according to claim l, wherein the primary conductor iscomposed of two tubular parts provided with cooling ns at the endsthereof facing each other.

5. A high frequency transformer comprising a rst substantiallycylindrical conducting member having an end, a second substantiallycylindrical conducting member having an end substantially abutting butnot electrically contacting the end of said first member, atleast one ofsaid members being hollow at its end whereby said abutting ends define acavity, an annular core of high frequency iron disposed within saidcavity, electrically conductive means extending from said first memberthrough said core to said second member to unite said two members intoone current conductor, and a winding wound on said cure to provide anelectromagnetic coupling to said current conductor.

HANS WEHRLIN.

